Cycles: Shadeless "Shadow-catcher" Material for Rendering onto Footage
Divide objects into 2 scene layers, move actors (Suzanne) onto layer 1, move backgrounds onto second layer, place lights into both of them.
Setup 3 render layers - First for layer 1, second for layer 2 and third for layer 2 with layer 1 excluded:





Render it with transparent bacgrounds, it will produce results like this:







In compositor divide render layer 2 (BG_With_Shadow) with 3 (BG_Clean) - you now have a shadow to multiply over your backdrop and layer 1 to composite into backdrop: